Continuously variable transmissions have been known for a number of years as a driving means for the road engaging wheels of a vehicle to provide smooth acceleration without the usual shifting of gears by clutches and brake bands in an automatic transmission. A continuously variable transmission conventionally includes a pair of variable sheave pulleys, an endless belt extending between and in driving engagement with the pulleys and control means to alter the effective pulley diameters and thus change the belt ratio during operation of the vehicle.
In automotive applications, a hydraulically-actuated clutch is required on the driven pulley shaft as a starting device and for rotation of a shaft leading to the forward/reverse gearing to drive the final differential for the road-engaging wheels. The starting clutch is positioned on the driven pulley shaft so that the pulleys can be constantly rotating in one direction and rotation is not interrupted when shifting from forward to reverse and vice versa during vehicle operation. Thus, initiating movement of the vehicle is accomplished by a speed responsive friction starting device wherein a clutch is controlled by the hydraulic control means for the pulleys.
Prior patents relating to hydraulically-actuated starting clutches cover a broad base of applications. One prior problem of such starting clutches resided in the centrifugally generated oil head pressure developed at the clutch piston during operation. The start clutch has a basic torque curve plus a centrifugally added torque as a result of the centrifugally induced oil pressure on the piston at elevated speeds. The chain belt/sheaves have a similar arrangement. In the past to keep the clutch torque (basic plus centrifugal torque) below the chain belt/sheave torque (basic plus centrifugal), balance weights were added to the clutch Belleville spring to counter the influence of the centrifugal torque of the clutch (see U.S. Pat. No. 4,520,912), or a shroud was positioned either extending inwardly from the outer periphery of the clutch cover, as shown in U.S. Pat. No. 4,699,259, or at least partially covering the open end of the piston chamber leading into the actuating chamber for the clutch piston, such as seen in U.S. Pat. No. 4,556,556. This would keep the total torque of the clutch below the chain belt/sheave torque. Another problem was to provide a sufficient supply of cooling oil in the clutch compartment to adequately cool the friction surfaces of the clutch plate, especially during engagement and disengagement of the clutch where slippage occurs.